System and method for fabricating miniature structures on a flexible substrate

ABSTRACT

A system for the fabrication of patterned miniature structures, such integrated circuits, includes a continuous, flexible substrate that is transported by rollers to a series of processing stations. To ensure proper alignment amongst the various stations, the substrate is provided with at least one fiducial that is raised above its top surface a height that maximizes optical contrast when viewed interferometrically. At least one processing station includes an optical device that is capable of both interferometrically identifying the fiducial for alignment purposes and subsequently illuminating the substrate with a modifiable light pattern as part of a photolithographic process. Fiducials can also be used to identify gross geometric variances in the substrate caused by external factors, such as heat and moisture. In turn, a web adjustment element can be used to apply selective heat or tension to the substrate in order to correct such geometric variances.

FIELD OF THE INVENTION

The present invention relates generally to the fabrication of miniaturestructures and, more particularly, to the fabrication of miniaturestructures on flexible substrates.

BACKGROUND OF THE INVENTION

The miniaturization of technological structures is an ever-increasingtrend across a wide range of fields including, but not limited to,electronics, biotechnology and electro-optics. In the art, thefabrication of miniaturized technological structures is commonlyreferred to as microfabrication when used to manufacture structuresmeasured in microns (10⁻⁶ m) and nanofabrication when used tomanufacture structures measured in nanometers (10⁻⁹ m) or smaller. Ascan be appreciated, it has been found that the resultant size ofminiaturized structures is often limited by constraints associated withthe particular fabrication process utilized to construct suchstructures.

The fabrication of miniature electrical devices, such as integratedcircuits, is most commonly achieved using a multi-stepped, lithographicprocess in which patterned layers are sequentially formed onto a commonsubstrate in a stacked relationship. Specifically, as part of thefabrication process, a uniform layer of resist is typically depositedonto the top surface of a flattened substrate. Thereafter, eachminiaturized pattern is transferred into the layer of resist, forexample, through exposure to light directed through a patterned mask(i.e. photolithography) or through direct mechanical deformation (i.e.imprint lithography).

In photolithography, the optically exposed areas are reacted and thenthe resist is developed by rinsing it in a bath. When positivephotoresist is utilized, the reacted area becomes soluble and is rinsedaway. When negative photoresist is utilized, the unexposed area isrinsed away. Effectively, a positive or negative template is created,which is left on the surface of the substrate, through theabove-described exposure and development process. In a subsequent step,the entire surface is processed, for instance, by etching the surface,reacting the surface (e.g. as in doping to create a semiconductor),evaporating the surface, or depositing onto the surface, all in apatterned way through the photoresist template. Once the desiredpatterns are formed on the substrate, any remaining resist is thenremoved. In this manner, a plurality of miniature structures can beefficiently constructed onto a common substrate.

A semiconductor wafer (e.g. a silicon wafer) most commonly serves as thesubstrate on which miniature electrical devices are constructed usingfabrication techniques of the type as described above. As can beappreciated, semiconductor wafers are relatively rigid and stable innature and, as such, serve as a suitable construct on which to performthe various steps of the device manufacturing process.

However, it has becoming increasingly desirable in the art for miniaturestructures to be fabricated on thin, flexible substrates. The use of athinner, more flexible substrate introduces a number of notableadvantages over semiconductor wafers including, but not limited to, asignificant reduction in the device size scale (e.g. in thickness), anexpanded range of potential applications based on the flexibleconstruction of the device, as well as enhanced manufacturingcapabilities by incorporating the substrate as part of a continuous web,or roll.

Although desirable for the reasons set forth above, the use of thin,flexible substrates on which to fabricate miniature structuresintroduces a number of notable manufacturing challenges. In particular,it has been found that certain external factors, such as environmentalconditions, can greatly affect geometric aspects of the substrate. Forinstance, a flexible substrate constructed out of polyethyleneterephthalate (PET) (i) has a thermal coefficient of expansion which isapproximately 30 times greater than silicon, (ii) has stiffness which isapproximately 1/50^(th) the stiffness of silicon, and (iii) couldexperience a change in volume as great as 0.5% upon exposure to moisturewhereas, under similar conditions, silicon would not experience a changein volume.

As a result, a thin, flexible substrate is prone to stretch, contract,warp or otherwise deform in one or more dimensions in response to directexposure to heat, moisture or tension. The creation of these types ofvariances in the structural form of the substrate can affect the levelof precision by which each pattern is formed, largely due to issues inproperly aligning the substrate throughout the various fabricationstages. This lack of precision introduced into the fabrication processcan, in turn, significantly compromise the quality of the resultantproduct, especially as it relates to fabrication of nanometer-scalefeatures and designs.

A low cost, well-known solution for deterministically producing veryprecise features in a miniature structure is to fabricate the structurethrough a process known in the art as block copolymer (BCP)self-assembly. BCP self-assembly allows for the fabrication of flexiblestructures in various shapes of micron to nanometer feature size bylinking molecules together according to molecular weight and stressbiases over a smooth or an embossed surface (e.g. through theapplication of a coating which is then evaporated or developed underheat or other actinide energy). The aforementioned process therebyenables the structure to undergo self-assembly in a manner that isself-consistent but not connected to any macro feature (i.e. not wiredto the outside world). However, they may be aligned and orientedrelative to an embossed surface on which they were developed or grown.

Although well-known in the art, the fabrication of miniature structuresusing block copolymer self-assembly often requires means for physicallyconnecting the structure to a larger deterministic circuit in order toallow for the delivery of power and/or communication signalstherebetween. As a result of this connection requirement, the entirefabrication process is rendered considerably more complex and may renderthe microfabrication useless unless some means to wire and align theself-assembled parts to the macro world is achieved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new and improvedsystem and method for fabricating miniature structures on a flexiblesubstrate.

It is another object of the present invention to provide a system andmethod for fabricating miniature structures on a flexible substrate witha high level of precision.

It is yet another object of the present invention to provide a systemand method as described above that allows for the fabrication ofminiature structures at a microscopic or nanoscopic scale.

It is still another object of the present invention to provide a systemand method as described above that detects and compensates for variancesin the geometric aspects of the substrate caused by external factors,such as environmental conditions.

It is yet still another object of the present invention to provide asystem as described above that has a limited number of parts, isinexpensive to implement, and is easy to use.

Accordingly, as one feature of the present invention, there is provideda system for fabricating miniature structures, the system comprising (a)a flexible substrate on which the miniature structures are fabricated,the flexible substrate comprising a top surface, a bottom surface, and afiducial, the fiducial having an reference surface that lies in adifferent plane than the top surface, and (b) an optical device forilluminating the flexible substrate with a source light tointerferometrically detect information relating to the fiducial, thesource light being of a first wavelength and a first amplitude.

Various other features and advantages will appear from the descriptionto follow. In the description, reference is made to the accompanyingdrawings which form a part thereof, and in which is shown by way ofillustration, various embodiments for practicing the invention. Theembodiments will be described in sufficient detail to enable thoseskilled in the art to practice the invention, and it is to be understoodthat other embodiments may be utilized and that structural changes maybe made without departing from the scope of the invention. The followingdetailed description is therefore, not to be taken in a limiting sense,and the scope of the present invention is best defined by the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference numerals represent like parts:

FIG. 1 is a front perspective view of a system for fabricating miniaturestructures on a flexible substrate, the system being constructedaccording to the teachings of the present invention;

FIG. 2 is an enlarged, fragmentary, section view of a sample segment ofthe flexible substrate shown in FIG. 1;

FIG. 3 is a top view of a sample section of the flexible substrate shownin FIG. 1 when viewed interferometrically under ideal conditions;

FIG. 4 is an enlarged, perspective view of one of the optical devicesshown in FIG. 1 with selected components of the optical device beingshown in section, the optical device being shown in relation to the webas transported by rollers, the rear surface of the web being shownsupported by a planar member;

FIG. 5 is a simplified top plan view of a sample section of thesubstrate shown in FIG. 1, the substrate being shown provided anarrangement of fiducials;

FIG. 6 is a simplified top plan view of the sample section of thesubstrate shown in FIG. 5, the substrate being shown in position above aweb adjustment element that utilizes tension to correct gross geometricvariances, the web adjustment element being shown in dashed form; and

FIG. 7 is a simplified top plan view of the sample section of thesubstrate shown in FIG. 5, the substrate being shown in position above aweb adjustment element that utilizes heat to correct gross geometricvariances, the web adjustment element being shown in dashed form.

DETAILED DESCRIPTION OF THE INVENTION System 11 for the Fabrication ofMiniature Structures

Referring now to FIG. 1, there is shown a first embodiment of a systemfor fabricating miniature structures on a flexible substrate, the systembeing constructed according to the teachings of the present inventionand identified generally by reference numeral 11. As will be explainedfurther in detail below, system 11 is specifically designed to detectand compensate for variances in the geometric aspects of the flexiblesubstrate caused by external factors, such as environmental conditions,thereby ensuring proper alignment of the flexible substrate throughoutthe various stages of the fabrication process.

System 11 comprises a continuous substrate, or web, 13 that issequentially transported by rollers 15-1 thru 15-3 to a series ofprocessing stations 17-1 thru 17-6. Together, stations 17 areresponsible for, inter alia, patterning layers of material on substrate13 in such a manner so as to yield the desired miniature structure.

As defined herein, miniature structure denotes any device or feature ofa limited size (e.g. at micrometer scales, nanometer scales or smaller)that is fabricated on a substrate through a sequence of patterning stepswhich rely upon, inter alia, embossing, optical masking, and/or imageprojection. For instance, a miniature structure encompasses, but is notlimited to, an integrated circuit for a semiconductor device, such as atransistor or a microprocessor.

As will be explained further below, each of processing stations 17-1thru 17-4 is provided with an optical device 19 that is capable of notonly directing light onto substrate 13 in a particular pattern (i.e. aspart of a photolithographic patterning step) but also detecting one ormore fiducials (i.e. fixed bases of comparison, or markers) provided onsubstrate 13. By detecting fiducials on substrate 13 at various stagesof the fabrication path, proper alignment of substrate 13 can be ensuredamong the series of processing stations 17, thereby allowing for thefabrication of very precise, small scale structures, which is aprincipal object of the present invention.

Most notably, the information received through the detection offiducials by each optical device 19 can be used to identify largergeometric variances in substrate 13 (e.g. distortion or non-uniformscaling) caused by external factors, such as heat, moisture and thelike. To compensate for the presence of a geometric variance insubstrate 13, a web adjustment element 21 is preferably provided alongthe fabrication path to selectively treat substrate 13 in such a mannerto eliminate the variance (i.e. to restore substrate 13 to its originalgeometry), as will be explained further below. In this manner, system 11is adequately designed not only to adjust for undesired spatialvariances (i.e. changes in position or orientation) of substrate 13relative to each processing station 17 but also to correct for undesiredgeometric variances (i.e. changes in the overall geometry) in thesubstrate 13 itself caused from exposure to certain external factors. Assuch, system 11 is constructed to effectively compensate for commonalignment challenges that are introduced through the use of a substrate13 that is both thin and flexible in nature.

Construction of Substrate 13

Substrate 13 is preferably constructed as a continuous web of a thin,flexible material, such as a ribbon of polymer material. As referencedabove, the thin, flexible construction of substrate 13 streamlines thefabrication process and allows for the production of structures that areboth limited in size and available for use in a wide range of potentialapplications, which is highly desirable.

As seen in FIG. 2, substrate 13 includes a base layer 23 and a highlyreflective reference layer 25 that are formed together to yield aunitary, continuous strip. As can be seen, reference layer 25 includes agenerally flat bottom surface 27 and a generally flat top surface 29. Asa feature of the present invention, at least one fiducial 31 is providedin reference layer 25 and serves as an easily identifiable, fixed basisof reference, or comparison, throughout the fabrication process.

Fiducial 31 is represented herein as a relief structure, generallytrapezoidal in transverse cross-section, that is raised above topsurface 29 a defined height H. Formed as such, fiducial 31 defines anexposed, generally planar reference surface 33 which has a fixed widthW.

It is to be understood that fiducial 31 could be formed into referencelayer 25 using known nanoimprint lithographic techniques. As such, therelative size of each fiducial 31 could be constructed at micron, oreven submicron, levels, thereby limiting its impact on the overallfootprint of substrate 13.

As can be seen, reference surface 33 of fiducial 31 lies in a differentplane than top surface 29. As a principal feature of the presentinvention, the multi-planar construction of reference layer 25introduces a spatial phase delay into a source light L_(s) illuminatedonto substrate 13. This phase delay can, in turn, be used to maximizethe optical contrast between surfaces 29 and 33 and thereby enablelithographic-style optical devices 19 to effectively function as webalignment instruments.

Specifically, source light L_(s), with a wavelength λ and an amplitudeA, illuminated onto reference layer 25 is reflected therefrom with aphase delay that is dependent upon the height differential betweensurfaces 29 and 33. Accordingly, by defining height H as one-quarter thewavelength of source light L_(s) (i.e. λ/4), light L′ is reflected fromsurface 33 out of phase by 180 degrees (i.e. by λ/2, or π) in relationto light L″ reflected from surface 29, the reduction in height H byone-half of it being required since the light has to travel from opticaldevice 19 to substrate 13 and, in turn, back to optical device 19.

By combining the reflected light L′ and L″ with an unaltered referencelight L_(R) from the same light source (i.e. device 19), principles ofconstructive and destructive interference can be used to maximize thecontrast between light L′ and L″. In other words, constructiveinterference will cause reference light L_(R) to double the amplitude,or brightness, of any reflected light in phase therewith. At the sametime, destructive interference will cause reference light L_(R) tocancel (i.e. effectively eliminate) the brightness of any reflectedlight which is 180 degrees out of phase therewith.

As a result, when viewed interferometrically (i.e. by optical device19), alignment fiducials 31 can be detected relative to top surface 29with maximum optical contrast. Consequently, when viewed relative to areference waveform or beam L_(R), the substrate background (i.e. surface29) appears black when surface 33 appears white, and vice versa.Additionally, by utilizing a fiducial 31 with a width W at least asgreat as λ/2A, the maximum amplitude, or brightness, of the white lightdetected by optical device 19 is possible in view of the resolution ofits objective optics. As such, it is to be understood that the uniquetopography of the resultant web 13 enables optical devices 19 to readilyidentify fiducials 19 and, in turn, assist in the correction of anyspatial or geometric variances in substrate 13, thereby ensuring properalignment of substrate 13 at each processing station 17.

It should be noted that reference layer 25 need not be constructed of ahighly reflective material (or applied with reflective coating) in orderto achieve the desired interferometric contrast between fiducial 31 andtop surface, or background, 29. Rather, it is to be understood that thedesired interferometric contract between surfaces 29 and 33 could besimilarly achieved using a reference layer 25 that is not highlyreflective. In this situation, source light L_(s) could reflect of topsurface 23-1 of base layer 23 instead of reference layer 25.

In order to achieve the same level of contrast between fiducial 31 andtop surface 29 when a non-reflective reference layer 25 is utilized, theindex of refraction N of reference layer 25 needs to be incorporatedinto the geometric aspects of fiducial 31. Specifically, height H offiducial 31 is calculated as λ/(4*N) and width W is preferablycalculated as at least as great as λ/(2*NA), with the index ofrefraction N for reference layer 25 preferably falling within the rangeof 1.3 to 2.0. Additionally, it is to be understood that thereflectivity of the reference mirror in optical device 19 preferablymatches the reflectivity of reference layer 25 to ensure maximumconstructive and destructive interference with the reflected sourcelight.

It should also be noted that fiducial 31 need not be raised above topsurface 29 to achieve maximum optical contrast. Rather, it is to beunderstood that fiducial 31 could be recessed below top surface 29 at adepth of λ/4, when a highly reflective reference layer 25 is utilized,in order to achieve a similar phase change of 180 degrees.

In the present invention, the particular number, shape and arrangementof fiducials 31 on substrate 13 could be modified to suit the particularneeds of the fabrication process. Referring now to FIG. 3, there isshown a top view of a section of a sample substrate 113 when viewedinterferometrically under ideal conditions (e.g. with no tilt present).As can be seen, substrate 113 is provided with a plurality of fiducials131, each fiducial 131 being raised above the background surface at aheight that achieves maximum contrast, as detailed above.

As can be seen, fiducials 131 are shown in a variety of differentconfigurations. For instance, an elongated, linear, bar-like fiducial131-1 is provided which extends in parallel, preferably in continuum,along one side edge 113-1 of substrate 113. As such, fiducial 131-1could serve as a guide marker that can be used to ensure that web 113travels along a consistent linear track.

A pair of cross-type fiducials 131-2 and a pair of flattened circular,or centroid-type, fiducials 131-3 is additionally provided on substrate113. The use of a pair of complementary fiducials (e.g. fiducials 131-2and 131-3) allows for alignment correction across either a larger, orglobal, scale or a smaller, or local, scale, with one of the fiducialpair being compared against known coordinates to evaluate misalignmentin both the X and Y directions and the other of the fiducial pair beingcompared against the first fiducial to evaluate yaw, pitch and skew-typemisalignment.

It should be noted that centroid-type fiducials 131-3 are preferred overmost fiducial configurations since the 360 degree symmetrical nature ofcentroid-type fiducials 131-3 renders such fiducials 131-3 lesssensitive to the orientation of optical device 19 (more specifically,the orientation of the pixels for the camera in device 19), therebyproviding consistently higher resolution positioning data. Notably,centroid fiducials 131-3 provide 1/SQRT(N) sub-pixel resolution, where Nis the full number of circumference pixels. By comparison, a straightline-type fiducial (e.g. fiducial 131-1) would have a relatively large,discrete pixel resolution of 1 pixel, since would take a translation of1 full pixel width until the state of another pixel changes. However, itshould be noted that the pixel resolution of a straight line-typefiducial could be improved simply by intentionally incorporating tiltinto optical device 19, wherein a line-type fiducial with a tilt of 1/nwould cause a pixel change state for every 1/n translation.

Lastly, a barcode-type fiducial 131-4 is provided on substrate 113, witheach bar (or, in the alternative, the space between successive bars)being raised above the background of substrate 113 at the desired heightto ensure maximum optical contrast (e.g. λ/4 for a fully-reflectivereference layer). As can be appreciated, the design of fiducial 131-4 inthe form of a barcode enables a unique code to be associated withsubstrate 113, which in turn can be used to identify, inter alia, theminiature structure being constructed or some other useful aspectpertaining to the field of view.

As will be explained further below, the utilization of a large quantityof fiducials 131 at various locations on substrate 113 can help identifyboth global and local distortions caused by external factors, such asheat and moisture. It is also important to note that fiducials 131 canbe detected by optical devices 19, thereby eliminating the need forseparate alignment cameras, even if a degree of tilt is present betweensubstrate 113 and each optical device 19 (i.e. if the illumination pathis not perfectly perpendicular to the surface of substrate 113).

Construction of Optical Device 19

As referenced above, each optical device 19 is preferably constructed asa hybrid of (i) a photolithography instrument that is capable directinglight onto substrate 13 in a particular pattern, and (ii) aninterferometer for optically detecting raised fiducials 31, or othersurface patterns, on substrate 13 in order to ensure proper webalignment throughout the manufacturing process. By incorporating bothfeatures into a single instrument, the total number of componentsrequired for system 11 is lessened and, at the same time, the rate andaccuracy in fabricating multi-layered structures on substrate 13 isimproved. Accordingly, the particular construction of optical device 19and its intended use within system 11 serves as a principal novelfeature of the present invention.

Referring now to FIG. 4, there is shown a front perspective view,represented partially in section, of an optical device 19 shown inrelation to a web 13 transported at nominal tension by a pair of rollers15, the rear surface of web 13 being shown supported by a planar member211 that maintains web 13 flat and in its proper plane relative todevice 19. Preferably, member 211 is constructed of a porous material oris provided with transverse holes to permit the delivery of either (i)pressurized air to ensure frictionless contact between member 211 andthe rear surface of substrate 13 during web advancement or (ii) vacuumforces to retain substrate 13 firmly against member 211 to secure theweb from moving (e.g. during a patterning step).

Optical device 19 comprises a motor-driven, movable mounting plate, orstage, 213 on which is disposed an illumination device, or lamp, 215 forsupplying a source light (e.g. white light), an interferometricobjective 217 for generating a test light beam and a reference lightbeam from the light source, and an imaging device, or camera, 219 fordetecting the test and reference beams upon recombination, which in turncan be used to produce a surface map or otherwise extract surfacemeasurement parameters for substrate 13. In this manner, lamp 215,objective 217, and camera 219 together function as an interferometerthat can be used, inter alia, to identify fiducials 19 for alignmentpurposes, as will be explained further below.

Additionally, optical device 19 includes a spatial light modulator (SLM)221 that can be used to spatially modulate light produced from lamp 215.In this manner, modulator 221 can project, or expose, a particularpattern of light onto a photo-sensitive layer on substrate 13 relativeto fiducials 19 as part of a photolithographic process, as will beexplained further below.

Illumination device 215 is preferably of the type that can provideillumination energy that can be used for both interferometric andphotolithographic processes. Examples of light sources suitable for useas illumination device 215 include, but are not limited to, a mercurybulb, a laser source, or an ultraviolet (UV) light emitting diode (LED)(e.g. of the type sold by Luminus Devices, Inc., under model numberCBT12-UV).

As can be seen, interferometric objective 217 comprises a condenser lens223 that concentrates the source light produced by illumination device215 (in the patterned region defined by SLM 221) and a beam splitter 225that reflects the majority of the concentrated source light as (i) atest beam that is projected towards the test surface (i.e. substrate 13)within the SLM-defined region, and (ii) a reference beam, identical inintensity to the test beam, which is projected towards a partiallyreflective reference mirror 227 within the optical path. The test beamreflects off the test surface and recombines with the reference beamafter the reference beam similarly reflects off reference mirror 228.

A portion of the recombined beams passes through splitter 225 and iscollected by a tubus lens 229 for imaging onto a planar pixel array forcamera 219. The pixilation information is then used with appropriatephase shifting software to map and measure surfaces with sub-nanometervertical precision.

As will be explained further below, stage 213 is preferably designed tomove in multiple dimensions to rectify both gross and acute errors inalignment between substrate 13 and device 19. Specifically, stage 213 ispreferably able to move under motor control (i) in a transverserelationship relative to the advancement path of substrate 13, (ii)vertically in the focus direction, as well as (iii) rotationally amongmultiple axes so that fringes may be nulled and the image patterned ontosubstrate 13 is uniformly and crisply in focus (i.e. not distorted orotherwise out of focus).

As referenced briefly above, optical device 19 includes a spatial lightmodulator 221 that can be used to spatially modulate the intensity oflight produced from lamp 215. SLM 221 can be designed to be (i) eitherlight transmissive or reflective, and (ii) either controllable or static(e.g., in the form of photomask). Preferably, SLM 221 is of thecontrollable variety (e.g. an SLM of the type sold by Texas InstrumentsIncorporated under the DLP® DISCOVERY line of spatial light modulators)in order to allow for (i) the illumination of continuous light patternswhich are larger than the field of view of the modulator and (ii)surface probing without exposing. As a result, SLM 221 could be utilizedto illuminate a very small portion of the field (i.e. where fiducials 19are generally located), thereby preventing the exposure of light onto alayer of photoresist on substrate 13 until proper alignment isfinalized. In addition, a controllable SLM 221 enables surface mappingof small subsections of the field of view (which may be independentlyout of focus due to optical errors or the underlying topography ofsubstrate 13) in order to yield a composite exposure that is uniformlyin focus.

Alignment Method for Patterning on a Flexible Substrate

System 11 can be used in the following manner to pattern multiple layersonto a flexible substrate 13, with patterning precision ensured througha novel alignment method that relies upon the registration of alignmentfiducials 31 on substrate 13. Specifically, referring back to FIG. 1,continuous web-like substrate 13 is shown advanced by rollers 15-1 thru15-3 to a series of processing stations 17-1 thru 17-6. Together,stations 17 are responsible for, inter alia, patterning layers ofmaterial on substrate 13 in such a manner so as to yield the desiredminiature structure.

As referenced above, each optical device 19 is designed tointerferometrically detect fiducials 31 on substrate 13 for alignmentpurposes. Accordingly, as a first step in the novel fabrication process,it is envisioned that rollers 15 advance substrate 13 to at least oneoptical device 19 for global examination in order to detect any gross,large-scale variances in the overall geometry of substrate 13 as aresult of external factors (e.g. tension, moisture, heat and the like).Once in place relative to optical device 19, substrate 13 is stopped byrollers 15 and held stable at normal tension, preferably with a vacuumapplied thereto (e.g. through a planar support member 211) to ensurethat substrate 13 is maintained in a fixed, generally planarorientation. Thereafter, an optical device 19 in alignment therewith canbe used to globally scan substrate 13 and detect any gross variances inits overall geometry.

Referring now to FIG. 5, there is shown a simplified top plan view of asample section of a substrate 313 which includes an arrangement ofglobal fiducials 331 useful in the detection of gross geometricvariances. Specifically, substrate 313 is provided with a pair oflongitudinal side member fiducials 331-1 and 333-2 extending alongopposing sides of substrate 313, a plurality of cross member fiducials331-3 thru 331-10 which extend transversely between fiducials 331-1 and331-2, and a partitioning member fiducial 331-11 which extendslongitudinally across the approximate midpoint of fiducials 331-6 thru331-10.

For simplicity purposes only, each fiducial 331 is represented herein asbeing in the form of a bar-like fiducial (e.g. of the type shown asfiducial 131-1 in FIG. 4). However, it is to be understood that eachfiducial 331 need not be limited to a bar-like configuration, butrather, could include one or more alternately shaped fiducials arrangedin the same general alignment. For instance, it is preferred thatcentroid-like fiducials (e.g. of the type shown as fiducial 131-3 inFIG. 4) be located at the junction of transecting fiducials 331 and, inaddition, at the midpoint of each bar-like fiducial 331 to detect anyweb narrowing.

Together, fiducials 331 define, or frame, a series of larger exposurefields 333-1 thru 333-3 and a series of smaller exposure fields 335-1thru 335-8. As can be appreciated, sub-global fiducials (not shown) maybe placed within smaller exposure fields 335 to provide fine targetedalignment (i.e. alignment within one particular smaller field 335 andnot a greater section of substrate 313).

In the present example, side member fiducials 331-1 and 331-2 extend ina non-parallel relationship (e.g. due to non-uniform lateral strainapplied to substrate 313 by external factors). As can be appreciated,this gross distortion of substrate 313 can significantly compromise theaccuracy of subsequent patterning steps.

Consequently, a coarse scan of fiducials 331 by optical device 19 woulddetect the undesired geometric strain on substrate 313. In responsethereto, a processor (not shown), which is programmed with the necessarysoftware, would cause web adjustment element 21 to treat substrate 313in such a manner to eliminate the variance (i.e. to restore substrate 13to its original geometry with fiducials 331-1 and 331-2 arrangedsubstantially in a parallel).

For example, web adjustment element 21 may utilize tension to correctgross geometric variances in substrate 313. Referring now to FIG. 6,substrate 313 in shown in relation to a web adjustment element 421 thatutilizes tension to correct gross geometric variances. Web adjustmentelement 421 is preferably mounted on a polished smooth support element423 and includes a pair of transverse tension bars 425-1 and 425-2, aseries of longitudinal tension bars 425-3 thru 425-5, and a singleopposing tension bar 425-6. Together, tension bars 425 are arranged inalignment around the periphery of one large exposure field 333-1 and, assuch, can be used to restore the desired geometry of that particularfield 333-1 (i.e. rather than the entirety of substrate 313).

Web treatment may be performed by gripping substrate 313 with transversetension bars 425-1 and 425-2 (e.g. using clamping and/or vacuum forces).Tension bars 425-1 and 425-2 are then displaced, as needed, to correctfor global (i.e. large-scale) longitudinal errors (i.e. variances withinfield 333-1 in relation to the longitudinal axis) that are detectedthrough inspection of side fiducials 331-1 and 331-2.

Thereafter, tension bars 425-3 thru 425-6 are similarly utilized tocheck the transverse geometric aspects of field 333-1, since the widthof field 333-1 (as determined through inspection of cross fiducials331-3 and 331-4 is affected proportionally to Poisson's ration. Usingtension bar 425-6 as the datum bar, one or more of tension bars 425-3may selectively apply localized displacement of substrate 313 to correctfor any bowing or other similar variance.

As another example, web adjustment element 21 may utilize heat tocorrect gross geometric variances in substrate 313, which is preferredover the use of tension since there is less cumulative stress applied tosubstrate 313. Referring now to FIG. 7, there is shown substrate 313 inrelation to a web adjustment element 521 that utilizes heat to correctgross geometric variances.

As can be seen, web adjustment element 521 is similar to web adjustmentelement 421 in that web adjustment element 521 includes a pair oftransverse tension bars 525-1 and 525-2 as well as a longitudinaltension bar 525-3 that are arranged around the periphery of three sidesof one large exposure field 333-1. Web adjustment element 521 differsfrom web adjustment element 421 in that web adjustment element 521includes three, equal-sized, heating plates 527-1 thru 527-3 that arearranged as three distinct zones within exposure field 333-1. In use,each plate 527 can be set at a specified temperature, the variance intemperature between plates 527 causing the dimensions of substrate 313within exposure field 333-1 to change accordingly. Accordingly, if heatis applied unevenly by plates 527, one side of substrate 313 wouldexpand to a larger extent than the other, thereby resulting in curvaturein substrate 313. Imparting curvature into substrate 313 can be usedintentionally to correct distortion as well as direct substrate 313along a correspondingly curved track.

With gross geometric variances in substrate 313 corrected in the mannerset forth above, each optical device 19 is then adjusted to compensatefor the presence of tilt fringes. As can be appreciated, tilt fringesare interferometerically observed in a plane surface when the test beamprovided by optical device 19 projects in a non-orthogonal relationshiprelative to substrate 313. This angular offset, or tilt, of the testbeam causes both constructive and destructive interference which, inturn, results in the generation of an alternating pattern of light anddark fringes.

The period of the fringe pattern (i.e. the distance between eithersuccessive constructive fringes or successive destructive fringes) canbe measured by optical device 19 and, in turn, used to calculate thetilt angle θ, since the fringe period P=λ/2 sin θ. Accordingly, thecalculated information can be used to adjust optical device 19 (viastage 213) so as to remove the presence of tilt. With optical device 19properly oriented relative to substrate 13, a patterned image would beprojected by optical device 19 at a right angle relative to an exposedphotoresist layer on substrate 13 and therefore be uniformly in focus,which is highly desirable.

After adjusting the tilt angle of each optical device 19 in the mannerset forth above, optical device 19 can be used to pattern photoresist onsubstrate 13 in the following manner. First, global fiducials 31 onsubstrate 13 are located by optical device 19 and the X, Y, and Zcoordinates for each fiducial 31 are recorded. Using the recordedcoordinates for each fiducial, the coordinate axes for each opticaldevice 19 can be calculated and scaled, as needed, in order to properlycalibrate each patterning instrument. In the alternative, rather thanrecalibrate the orientation of the axes for each optical device 19, itis to be understood that the desired image to be projected by opticaldevice 19 could be distorted to compensate for any misalignment relativeto web 13.

Upon completion of the gross adjustment of each optical device 19, fineadjustment of each device 19 is achieved by projecting a test image(e.g. a point) relative to one or more fiducials 31. The location of thetest image (relative to the designated fiducials 31) on spatial lightmodulator 402 is compared against the location of the test image(relative to the designated fiducials 31) measured on substrate 13 usinginterferometry. Any offset in the location of the test image is recordedand, in turn, utilized to modify the pattern on SLM 402 in accordancetherewith. With the pattern adjusted to compensate for any acutealignment discrepancy, photolithographic patterning of substrate 13 canbe performed.

It should be noted that acute adjustment of optical device 19 using aprojected test image initially requires the Z axis needs to be scannedvertically through π/2 or λ/4 in order to identify and record thebrightest pixels. Recordation of the brightest pixels is requiredbecause destructive interference will null portions of the image.However, by shifting the brightest pixels by π, any destructiveinterference will become constructive interference and, accordingly, thenull portions will be eliminated. The actual location of the brightimage on web 13 should then be compared against the location of thecorresponding image on SLM 402 to compute the acute offset or overlayerror. The optical column for device 19 should be translated accordinglyand/or SLM pixels shifted until minimal error is achieved.

Referring back to FIG. 1, stations 17-1 thru 17-4 are capable ofphotolithographic patterning of web-like substrate 13 in the manner setforth in detail above. In addition to photolithographic patterning,system 11 is also capable of imprint lithographic patterning, with allpatterned layers being properly aligned through detection of fiducials31.

In particular, station 17-5 provides system 11 with imprint lithographycapabilities. Specifically, station 17-5 includes an embossing roller601 which cooperates with pinch roller 15-2 to emboss web 13 with adefined pattern. Using the fiducial information collected by opticaldevices 19, strain and/or thermal correction can be applied to web 13prior to embossing by roller 601.

It should be noted that, contrary to the construction of traditionalimprint lithography systems, web 13 wraps partially around pinch roller15-2, but is limited to a linear region of contact with embossing roller601. This construction is required in order to minimize the retentiveforce applied onto web 13 at station 17-5 and thereby allow forselective web geometry correction, as detailed above.

Traditional wetting and gelling of polymer web 13 is not possible in thepresent construction since the process causes adhesion in the web, whichmakes subsequent web correction very difficult or even impossible at therequired resolution. Consequently, in order to wet embossing roller 601,material may be applied to roller 601 using an inkjet 603 or a transferroller (not shown). Another strategy for applying material to web 13 isto slowly steer embossing roller 601 at a rate that only slightlydistorts features fabricated on web 13 and, subsequent thereto, curingsuch distortions by applying light from a focused light source 605 (e.g.laser) outside of the field being embossed.

After completion of imprint lithography processes, web 13 is flood curedwith a flood lamp 607 at station 17-6. After the flood curing step, theprocess for fabricating miniature structures on flexible substrate 13 iscomplete.

In addition to alignment and patterning, an optical device 19 located ata post-patterning station (e.g. station 17-4) could be utilizedadditionally as inspection device with extremely sensitive, nanometerclass, vertical measurement capabilities. Accordingly, using the z-axisto phase modulate the inspection beam under low intensity illumination,device 19 can be utilized not only to measure the height of certainfeatures on web 13 (to determine and correct fabrication errors) butalso to detect dust or other contaminants which may compromise theresultant functionality of the fabricated structure.

The embodiments shown above are intended to be merely exemplary andthose skilled in the art shall be able to make numerous variations andmodifications to it without departing from the spirit of the presentinvention. All such variations and modifications are intended to bewithin the scope of the present invention as defined in the appendedclaims.

What is claimed is:
 1. A system for fabricating miniature structures,the system comprising: (a) a flexible substrate on which the miniaturestructures are fabricated, the flexible substrate comprising a topsurface, a bottom surface, and a fiducial, the fiducial having anreference surface that lies in a different plane than the top surface;and (b) an optical device for illuminating the flexible substrate with asource light to interferometrically detect information relating to thefiducial, the source light being of a first wavelength and a firstamplitude.
 2. The system as claimed in claim 1 wherein the substrate isconstructed as a continuous web of material.
 3. The system as claimed inclaim 2 wherein the substrate includes a reference layer formed onto abase layer, the reference layer including the top surface and thefiducial.
 4. The system as claimed in claim 3 wherein the referencelayer substantially reflects light illuminated thereon.
 5. The system asclaimed in claim 4 wherein the reference surface for the fiducialextends above the top surface a height that is approximately equal toone-quarter of the first wavelength.
 6. The system as claimed in claim 5wherein the reference surface has a width that is approximately equal tothe first wavelength divided by two times the first amplitude.
 7. Thesystem as claimed in claim 3 wherein the reference surface substantiallyrefracts light illuminated thereon, the reference surface having a firstindex of refraction.
 8. The system as claimed in claim 7 wherein thereference surface extends above the top surface of the flexiblesubstrate at a height that is approximately equal to the firstwavelength divided by one-quarter of the first index of refraction. 9.The system as claimed in claim 8 wherein the reference surface has awidth that is approximately equal to the first wavelength divided by twotimes the product of the first amplitude and the first index ofrefraction.
 10. The system as claimed in claim 1 wherein the referencesurface for the fiducial has an elongated linear configuration.
 11. Thesystem as claimed in claim 1 wherein the reference surface for thefiducial has a cross-shaped configuration.
 12. The system as claimed inclaim 1 wherein the reference surface for the fiducial has a circularconfiguration.
 13. The system as claimed in claim 1 wherein thereference surface for the fiducial is in the form of a barcode.
 14. Thesystem as claimed in claim 1 wherein the optical device is capable ofilluminating the flexible substrate with a source light having a patternthat is modifiable.
 15. The system as claimed in claim 14 wherein theoptical device comprises: (a) an illumination device for producing thesource light; (b) an interferometric objective for producing a test beamthat reflects off the substrate and a reference beam that reflects off areference mirror; and (c) a camera for detecting the test and referencebeams.
 16. The system as claimed in claim 15 wherein the illuminationdevice, interferometric objective and camera are mounted on a common,movable stage.
 17. The system as claimed in claim 16 further comprisinga spatial light modulator between the illumination device and theinterferometric objective for modulating the source light into amodifiable pattern.
 18. The system as claimed in claim 1 furthercomprising a web treatment device for varying a geometric aspect of theflexible substrate in view of the information relating to the fiducialdetected by the optical device.
 19. The system as claimed in claim 18wherein the web treatment device utilizes heat to vary the geometricaspect of the flexible substrate.
 20. The system as claimed in claim 19wherein the web treatment device utilizes tension to vary the geometricaspect of the flexible substrate.